1. Field of the Invention
This invention relates to a musical instrument, the slide guitar, specifically the means by which one may temporarily reduce the operating length of the guitar's strings so that the instrument can be played in a higher key by strategies similar to those used in a lower key in absence of said means.
2. Description of the Related Art
The main areas of a stringed musical instrument characterized by guitars, banjos and mandolins are the body, the neck and the head. The surface of the neck facing the strings is covered by a fretboard on which frets (narrow transverse elevations) are attached. The strings are positioned over the fretboard and frets by a bridge on the body and a nut at the head end of the fretboard. A capo is a device to reduce temporarily the operating length of the strings of such a musical instrument, so that the instrument can be played in a higher key by strategies similar to those used in a lower key in absence of a capo. For instruments, whose strings are pressed by the musician's fingers against frets during performance, a capo acts by clamping the strings against a fret (Averitt U.S. Pat. No. 620,560; Russell U.S. Pat. No. 1,788,636; Gould U.S. Pat. No. 4,304,165; Shubb U.S. Pat. No. 4,250,790; Cornette U.S. Pat. No. 4,621,558). The strings of a slide guitar (e.g. Dobro) are about 1 cm above the fretboard, so their operating length cannot be reduced practically by their being clamped against a fret.
Few slide-guitar capos have been patented or produced commercially. All of them involve at least: 1) a hard-surfaced bar approximately 6 cm long, to be positioned perpendicular to and across all the strings, either above the strings or between the strings and the fretboard; 2) a second bar (usually padded) of similar length to be positioned parallel to the first bar and on the other side of the strings from the first bar; and 3) a means whereby one bar presses the strings against the other bar, so that strings vibrate from their zone of contact with a hard edge of the first bar (the stop zone), rather than from the guitar's nut. This design has several desirable results. 1) The strings are thoroughly stopped at the stop zone to achieve durable ring and minimal buzz. 2) The strings are well damped behind the stop zone to avoid vibrations of the non-playing string segments between that zone and the guitar's nut. 3) Application of the capo produces only modest additional tension in the strings, so their pitches are only slightly above those expected from the location of the stop zone. 4) In the more thoughtful designs (which apparently exclude that of Dunlop U.S. Pat. No. 3,933,077), the capo can be placed at a position with the stop zone over the back edge of a fret or close by to achieve adequate tuning despite the effect of string tightening to raise pitch.
Most slide-guitar capos suffer from a major flaw. In those cases where a thick bar is above the strings and coincident with or near the stop zone (Hathcock U.S. Pat. No. 4,671,156; commercial make Leno; Dunlop U.S. Pat. No. 3,933,077; apparently unpatented design of Gene Wooten of Nashville, Tenn.), the capo interferes intolerably with the musician's operation of the slide bar at those frets near the capo. In the case where a thin bar is above the strings near the stop zone (Shubb U.S. Pat. No. 4,250,790, apparently a patent for the clamping mechanism; commercial make Shubb C6B), this problem is less severe but sill significant. The slide-guitar capo recently patented by Ellis (U.S. Pat. No. 5,284,077) solves this problem completely and will function well under most circumstances.
An unpatented and non-commercial slide-guitar capo consists of a hard, flat stick of dimensions between those of a tongue depressor and those of an ice-cream stick. This hard, flat stick is slipped between the fretboard and strings in the horizontal orientation and then rotated to the vertical orientation so as to press up on all strings at a zone just behind (on the nut side of) a fret. The advantage of this simple device by comparison to those just described is that their are no obstructions to impair the musician's operation of the slide bar at frets near the capo. It has several disadvantages. 1) The downward force of the strings is transmitted to and borne by a small area of fretboard, so their is a risk of scarring the fretboard, especially if the capo were of metal. 2) One would need at least three capos of this kind with differing widths, a greater width (dimension from fretboard to strings) being needed to stop the strings at higher fret positions, where the strings are further from the fretboard and require greater upward displacement to exert a given force against the capo. 3) The capo is not well stabilized between fretboard and strings, so it may move or collapse during performance. 4) Application of the capo adds significantly to string tension. 5) The non-playing segments of strings (between capo and nut) may not be adequately damped. 6) Ring duration may be shortened to an undesirable degree. 7) Buzzing may be difficult to avoid.